The conventional sensor is explosion-proof. It is placed into the liquid and measures the liquid resistance to a rotating cylinder. When the viscosity increases, then so does the liquid resistance, which will affect the speed of revolutions of the rotating cylinder. If the viscosity comes outside the set value then it is automatically translated to the explosion-proof electromagnetic solvent valve, upon which the solenoid valve is opened and solvent is added to the ink until the pre-set viscosity is reached again. The conventional sensor is safe and easy to use.